It is well known that frost conditions are bettered by the loss of heat through infra-red radiation of wave-lengths in the range of 8-12 microns from the earth's surface and crops growing thereon to the atmosphere. The heat loss occurs particularly on windless clear nights. The distribution of temperature as a function of height above the ground surface under the typical frost conditions described above is illustrated in FIG. 1, which indicates that the air temperature reaches a minimum at a height of 10-20 ft. above the ground surface. The layer of atmospheric air at which the temperature is at a minimum is commonly referred to as the "inversion layer."
It is known that heat losses to the atmosphere from the ground surface during clear and windless nights are in the range of 600,000 KCAL/hectare/hour.
A variety of techniques are known for combatting frost damage to crops. The most popular of these is the use of surface combusion heaters. These involve the disadvantage that a large number of heaters, typically 100 per hectare, must be used to provide good coverage and that the fuel requirements comprise a caloric content approximately four times that of the heat losses to the atmosphere per unit time. The labour involved in operating such a large number of heaters as well as the cost of fuel render frost protection by conventional techniques a very expensive operation.
An alternative technique for preventing frost damage to crops is to create turbulence in the atmosphere so as to force the inversion layer downwards adjacent the ground surface, and thus below the regions of the fruit trees on which growing fruit is to be found. Such turbulence can be provided by means of a large motor-operated fans or jets or by a combination of the heaters and fans. This system is generally believed to be impracticable due to the high cost involved.
Another alternative technique for protecting crops against frost damage is direct water sprinkling wherein the radiation losses to the atmosphere are partially compensated for by latent heat produced by freezing of the water drops. This technique suffers from the serious disadvantage that significants amounts of ice, having a conductivity four times that of water, are formed on the crops, for example, on the leaves of fruit trees. Therefore the ice thus formed enhances the cooling of the leaves, which is undesirable.
Smoke generators have also been used for preventing radiation losses, but these are not satisfactory due to the fact that the size of the smoke particles, 0.5-0.6 microns makes them completely transparent to infra-red radiation.
Apparatus is also known for providing freeze protection with man-made fog. As described in Sunshine State Agricultural Research Report Vol. 19, No. 1-2 of March, 1974 and in a brochure issued by Mee Industries Inc. 4939 North Earl Street, Rosemead, Calif. and entitled "Freeze Protection," a fog is generated using a series of nozzles each of which sprays water from a small hole under a pressure of 500-600 PSI. A temperature increase of 1.degree.-7.degree. F. is said to be provided. The nozzles are arranged in fixed locations along supply conduits which are mounted at desired intervals throughout a growing area.
In the above-referred to Sunshine State Agricultural Research Report there is found a description of a system which generates fog by evaporating water, recondensing and coating the fog droplets with a layer of cetyl alcohol to prevent them from evaporating. According to the research report, this technique generated fog particles having droplets of too small a size and insufficient density to significantly reduce the radiation heat losses encountered.